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  • 20 June 2016

NationWide Laboratories follow the Clinical and Laboratory Standards Institute (CLSI) guidelines 

NationWide Laboratories follow the Clinical and Laboratory Standards Institute (CLSI) guidelines on antimicrobial susceptibility testing (AST). The following are definitions of the terms used:


This category implies that isolates are inhibited by the typical blood and tissue concentrations of antimicrobial agents achieved using the recommended oral or parenteral dosage for the site of infection.


This category implies that isolates are not inhibited by the usually achievable concentrations of the agent at the usual dosage schedule, and/or that the isolates demonstrate zone sizes that fall in the range wherein specific microbial resistance mechanisms (e.g. β-lactamases) are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.


The intermediate category includes isolates with antimicrobial agent MICs* that approach blood and tissue levels that are usually attainable, and for which response rates may be lower than for susceptible isolates. The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated (e.g. quinolones and β-lactams in urine) or when a higher dosage then normal of a drug can be used (e.g. β-lactams).

*Minimum Inhibitory Concentration (MIC): The lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation.


For topical therapy the susceptibility data should be regarded only as a guide.  In vitro methods test against drug concentrations likely to be achieved using oral or parenteral medication. Topical therapy achieves local concentrations far greater than those tested, and resistance to topical agents may be overestimated. 


For susceptible isolates (S), the standard recommended dose and duration of therapy for the type of infection is expected to be effective. For isolates showing intermediate susceptibility, higher doses (if licensed or considered safe) may be successful. The following list of references may be helpful:

• BSAVA formulary

• BSAVA P.R.O.T.E.C.T.  See

• National Office of Animal Health (NOAH) website:

• Drug manufacturer 

• Target - The Antimicrobial Reference Guide to Effective Treatment. Available at Bayer DVM:

• Antimicrobial Use Guidelines for Treatment of Urinary Tract Disease in Dogs and Cats: Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases. J. Scott Weese, Joseph M. Blondeau, Dawn Boothe, Edward B. Breitschwerdt, Luca Guardabassi, Andrew Hillier, David H. Lloyd, Mark G. Papich, Shelley C. Rankin, John D. Turnidge, and Jane E. Sykes. Veterinary Medicine International  Volume 2011, Article ID 263768, 9 pages doi:10.4061/2011/263768

• Greene's Infectious Diseases of the Dog and Cat


Veterinary surgeons are encouraged to follow the guidance produced by BSAVA, BVA and VMD regarding the use of antimicrobials.  Initially, it is important to consider if the isolate is clinically significant, and other potential treatment strategies, particularly topical therapy and correction of any underlying pathology which might predispose to secondary bacterial infection. The presence of multi-drug resistance does not confirm that the isolate is involved in the pathology, and correlation with cytology, histology and tissue culture may all help to increase the confidence in significance. If, after considering these factors and the initial AST results, there are no suitable therapeutic choices then further testing may be possible.  Additional testing could include a full MIC panel for isolates from non-urinary tract sites or a smaller UTI panel for clinically significant lower urinary tract isolates. A member of our clinical pathology team will be happy to discuss the options for your patient. 

We are pleased to announce that we are introducing SensititreTM true MIC testing on all bacterial isolates from all sites initially on submissions to our Knutton Microbiology Laboratory.

MIC testing, as opposed to results from Kirby-Bauer disk diffusion methods, give a more accurate guide to whether an organism is likely to be clinically responsive to treatment with an antimicrobial. The accuracy of the testing, and its interpretation, is significantly improved over disk diffusion testing, not least because almost all the subjectivity inherent in the disk diffusion assay is removed.

Moreover, while interpretive data for all organism/drug combinations is not available for disk diffusion methods, there is considerably greater availability of MIC data.

Importantly, not only will the results be more accurate and meaningful, but there will be no effect on the already rapid turn-around time: >98% of all culture and sensitivity test requests  received at the Knutton facility are reported at 48 hours following receipt of the sample, including at weekends and Bank Holidays. 

This is important because, while many might consider microbiology to be a drawn-out process (and one that is often considered a poor relative of other branches of pathology), having access to results that are reported rapidly is paramount in aiding the clinician in choosing an appropriate antimicrobial for treatment, thereby supporting the principle of proper antimicrobial stewardship and helping prevent the development and spread of multi-drug resistant bacteria.

This methodology also utilises true MIC and not extrapolated (sometimes referred to as ‘predictive’) [MIC] results, which is essential in fighting antimicrobial resistance for a number of reasons, including:

 Greater sensitivity; this method provides better antimicrobial resistance tracking.

 It gives the best measure of antibacterial effect, and therefore assists with therapeutic antimicrobial choices.

 The micro-broth dilution test, which is the basis of this methodology, represents the ‘gold standard’ in MIC testing. 

 The assay provides superior quality and reproducibility, with accurate results first time and without the need for retests

 It is the methodology of choice for global surveillance programmes, including those coordinated through the (US) FDA-CVM, USDA & CDC.

We will be employing a two-tiered system, the first tailored to those drugs that clinicians are encouraged to use as first-line therapy. In the event a multi-drug resistant organism is isolated, testing against a panel of second- and third-line drugs will be routinely undertaken and reported at no additional cost.

If you require further information, please do not hesitate to contact us.